Problems underlying the field of optical disks include (1) realization of a high density and (2) realization of a high speed. As for the realization of a high speed, for example, a storage system included in a large-capacity server system is replaced with an optical disk drive, high-speed recording is requested. Moreover, high-speed recording is desired in case where data is downloaded over the Internet.
On the other hand, at present, a phase change optical disk such as a DVD-RAM is widely adopted as a rewritable optical disk. According to the phase change disk technology, data is recorded by extending control so that the molecular structure of a recording layer will be changed between crystalline and amorphous states. The recording method will be described below. Namely, laser light whose power is demodulated is irradiated to initialized crystal. The laser pulse is composed of recording power, bottom power, and erase power. Consequently, recrystallization deriving from recording is prevented. New data is recorded while a mark that has already been recorded before irradiation of laser powers is being erased. This method is called direct overwriting.
When data is recorded or overwritten on a rewritable medium at a high speed, an amorphous mark must be erased at an equivalent high linear velocity. Therefore, the speed of crystallization must be raised. “Optical Data Storage 2001” (p. 76-87, 2001) carried by the Proceedings of SPIE Vol. 4342 describes an example in which an optical system including a light source that emits laser light whose wavelength is 405 nm and an objective whose numerical aperture (NA) is 0.85 are used to perform high-speed recording equivalent to recording at 120 Mbps. Herein, a material produced by adding Ge and Sb to a eutectic composition of Sb70Te30 is used to produce a recording layer. Herein, Sb is characteristic of a high speed of crystallization. The larger an Sb content, the higher the speed of crystallization of the recording layer. This enables high-speed overwriting.
The above example employs a unique pattern according to which laser light is emitted for recording. In normal phase change recording, as shown in FIG. 4A, laser light to be used for recording is modulated into high-level power (recording power Pw), medium-level power (erase power Pe), and low-level power (bottom power Pb). The sum of the durations of the recording power Pw and erase power Pb contained in one laser pulse is set to 1 Tw (where Tw denotes a window width). The laser pulse is repeatedly irradiated according to the pattern in order to record data. Therefore, a recording pulse is composed of laser pulses and called a multi-pulse. According to this method, a mark whose length corresponds to nTw (where n denotes a positive integer) is recorded using (n-1) or (n-2) multi-pulses. In the case of a DVD-RAM on which the shortest mark whose length corresponds to 3 Tw is recorded, the mark of 3 Tw long is recorded using one pulse, and the mark of nTw long is recorded using (n-2) multi-pulses. Moreover, when the length of the shortest mark corresponds to 2 Tw, the mark of 2 Tw long is recorded using one pulse, and the mark of nTw long is recorded using (n-1) multi-pulses. On the other hand, a semiconductor laser is adopted as a light source for an optical disk apparatus, the rise or fall time of laser light has a finite value and is typically about 2 ns. A pulse whose duration is equal to or shorter than 4 ns assumes a triangular wave as shown in FIG. 4B. The laser power does not reach the Pw level, and the energy applied to the recording layer is insufficient. Consequently, a mark is not recorded successfully. This leads to degraded quality of recorded marks. According to the aforesaid literature, the duration of a multi-pulse is set to 2 Tw as shown in FIG. 4C. The power of a laser pulse is raised to the recording power level Pw in order to record a mark.
If the speed of crystallization of a medium cannot be raised, direct overwriting is hard to do. In this case, a method of performing erasure twice is adopted as described in Japanese Patent Laid-Open Nos. S60(1985)-185232, H1(1989)-184631, and H2(1990)-027525. The Japanese Patent Laid-Open No. S60(1985)-185232 describes an information reproducing apparatus in which: two lasers capable of being driven independently of each other emit light waves that are polarized in different directions; a beam splitter separates the two laser light waves from each other; and one of the two laser light waves is used for recording or reproduction and the other is used for erasure. The Japanese Patent Laid-Open No. H1(1989)-184631 describes that: existing information is erased (during the first turn of a disk) by homogenizing a recording layer with the energy of a single light spot given during the first irradiation; and information is recorded (during the second turn of the disk) by alternating the power of energy given during the second irradiation between a high power level and a medium power level. Moreover, the Japanese Patent Laid-Open No. H2(1990)-027525 describes that existing information is overwritten with new information while being erased during the first irradiation, and the new information is verified with a carrier-to-noise (C/N) ratio improved during the second irradiation.
According to a method referred to as a two-spots technique, light waves emitted from two mutually-independent lasers are irradiated to different positions on the same track on a medium. One of the light waves is used for erasure, and the other is used for recording or reproduction. Thus, erasure is performed reliably. The preceding spot of laser light is irradiated by applying, for example, a DC voltage, and used for erasure. The succeeding spot of laser light is modulated similarly to the recording power of a laser pulse used for normal phase change recording. The preceding spot melts a recording layer or may change the state of the recording layer into the amorphous state. In other words, a homogeneous amorphous band is formed on the track. This operation shall be called DC writing because data is recorded by forming an amorphous band with application of a DC voltage. When the recording layer is melted, an amorphous mark formed past is fully erased. When a recording spot having recording power Pw, intermediate power Pe that crystallizes the recording layer, and bottom power Pb passes through the amorphous band that results from DC writing, a mark is brought to an amorphous state and a space is brought to a crystalline state with the intermediate power Pe. Consequently, the same record pattern as the one produced by performing normal phase change recording is formed.
When the speed of crystallization of the recording layer is raised, the speed of crystallization remains high even in a place where the temperature is low or a room temperature. An amorphous mark formed for recording data is crystallized due to heat dissipated at the room temperature or heat dissipated from a medium that absorbs the reproducing light power Pr, whereby the mark disappears. In other words, the durability of reproducing light is degraded or the life of stored data is shortened. FIG. 5 indicates the results of measurement of a time t (−1 dB) required until the carrier level of light forming a mark decreases by 1 dB with the reproducing light power Pr set to 0.3 mW. Herein, a recording layer shall be made of GeSbTe, and an optical system employed emits laser light whose wavelength is 405 nm and includes an objective whose numerical aperture is 0.85. FIG. 5 graphically indicates a Sb content as a function of the time t. The measuring method is such that the reproducing light power Pr is set to a range from 0.5 mW to 0.7 mW, and the relationship between the elapsed time t and the decrease DV in the amplitude of light is measured. This measurement is performed with the reproducing light power Pr set to several power levels.
On the analogy of formula (1) concerning the reaction kinetics,v=v0 exp(−Ea/kT)  (1)
where v denotes a reaction rate, Ea denotes activation energy, k denotes a Boltzmann's constant, and T denotes temperature, formula (2) is drawn out.DV=A exp[−B/(Pr·t)]  (2)
where A and B denote a constant.
A and B in the formula (2) are worked out from the relationship between the measured time t and the amplitude decrease DV. Consequently, the time t (−1 dB) required until the amplitude decrease DV diminishes by 1 dB with the power Pr set to 0.3 mW is calculated. Laser light exhibits a Gaussian distribution, and heat moves time-sequentially. The temperature T and power Pr are not always proportional to each other. Herein, a discussion will proceed on the assumption that the temperature T and power Pr are approximately proportional to each other. As seen from FIG. 5, when the Sb content increases, the durability of reproducing light is degraded rapidly. In particular, when the atomic percentage of the Sb content is 86, the durability is degraded in several sec.
Based on the data of FIG. 5, the life of stored data is estimated on the assumption that a disk is placed at the room temperature. When reproducing light is irradiated to the disk, the temperature of the recording layer of the disk rises to about 100° C. For brevity's sake, a model described below is adopted. Namely, the diameter of the spot of reproducing light shall be approximately 0.45 μm. When the light spot passes through a point on the disk, the temperature at the point shall reach 100° C. During the other time, the temperature shall be low enough and crystallization shall not take place. The durability of the reproducing light is, as seen from the formula (1), determined with the exponential function of temperature. When the room temperature is 25° C. and the Sb content of the disk is 80%, the time required until the amplitude of the reproducing light decreases by 1 dB at the room temperature is one year or less. Therefore, when one year elapses, a mark formed for recording data is rapidly crystallized. Consequently, it becomes impossible to reproduce recorded data.
When the Sb content comes to about 80%, rewriting causes marked degradation. FIG. 6 indicates the relationship between the number of times of rewriting and the degree of modulation of light in a case where the light is irradiated to a disk, which has a recording layer whose Sb content is 80%, at a linear velocity of 1.5 m/s in order to form a mark so as to record data. When the number of times of rewriting is equal to or larger than 10, the degree of modulation is degraded. When the number of times of rewriting is 200, no mark is formed. In this state, when the linear velocity of the disk is raised and recording is performed, a phase-change mark is slightly formed. This implies that rewriting causes the speed of crystallization to change. This phenomenon is thought to attribute to the fact that Sb separates its phase from the phases of the other constituents of a recording layer. When the Sb content of GeSbTe is approximately 70%, the composition of GeSbTe is close to a eutectic composition and is therefore stable. If Sb is further added, GeSbTe having the eutectic composition and Sb which are stable may be separated from each other. This is because when the GeSbTe and Sb are separated from each other, they are thermodynamically stable. Consequently, rewriting causes a crystallization characteristic to change. The method of raising the speed of crystallization by increasing an Sb content confronts limitations when it is adopted as a method of raising a recording speed.
Furthermore, it is hard to control high-speed recording. The example in which the duration of a multi-pulse is set to 2 Tw has been described in relation to the related art. In this case, it is impossible to make the number of pulses constituting a multi-pulse, which is used to form a mark of 3 Tw long, different by one pulse from the number of pulses constituting a multi-pulse that is used to form a mark of 4 Tw long. This is because when the number of recording pulses constituting a multi-pulse that is used to form a mark of 4 Tw long is decreased by one, the resultant number of recording pulses creates the same pattern as the recording pulses constituting a multi-pulse that is used to a mark of 2 Tw long. The pattern created by the recording pulses that are used to form a mark of nTw long must be defined differently between when n denotes an even number and when n denotes an odd number. When the recording speed is further raised, Tw becomes equal to or smaller than 2 ns. In this case, the duration of a multi-pulse must be, for example, about 4 Tw. The number of recording pulses must be discussed more carefully. Besides, since the duration of the multi-pulse is longer than 2 Tw, when a format defining that the shortest mark length is 2 Tw is adopted, it is very hard to control recording of a mark having a length of 2 Tw. Moreover, according to the related art, the degradation in the quality of a recording pulse is, as indicated in FIG. 4C, avoided by adopting a pulse whose duration is 2 Tw. When the power of laser light rises or falls, crystallization of a recording layer is facilitated. Consequently, recrystallization occurring during recording to be achieved by forming a mark is intensified, and jitter is worsened.
According to the conventional two-spots technique, one optical disk drive requires two lasers, optical elements for the lasers, and control circuits that are associated with the respective lasers and used to align the optical elements. This leads to an increase in the cost of the optical disk drive. Moreover, since the number of parts or circuits increases, it becomes hard to design an optical head compactly. The invention disclosed in the Japanese Patent Laid-Open No. S60 (1985)-185232 employs an array of semiconductor lasers capable of being mutually independently driven. Laser light waves are polarized on different planes, whereby a disk drive is designed compactly. However, according to the invention, unless the planes of polarization on which the light waves emitted from two semiconductor lasers meet exactly at 90°, the spot of one laser light is invaded by the power of the other laser light. The two spots are no longer independent of each other. The requirements for the planes of polarization of the two laser light waves depend on the properties of a disk. Depending on the requirements, the array of semiconductor lasers serving as a light source becomes very expensive. Moreover, according to the invention, the two laser light waves are passed through different objectives and irradiated to a disk. Auto-focusing servo systems and tracking servo systems must be formed in association with the respective light waves. Consequently, two laser drivers and two servomechanisms are needed. This leads to an expensive disk drive. The Japanese Patent Laid-Open Nos. H1(1989)-194631 and H2(1990)-27525 have revealed the method that laser light is irradiated to the same track twice. The first irradiation of laser light is used for erasure, and the second irradiation of laser light is used for recording. This is intended to improve the ratio of erasure efficiency to recording efficiency. However, the method requires twice more time for recording and cannot meet the need for high-speed recording.